1. ld of the Invention
The present invention relates generally to a driving circuit for a brushless DC motor and, more particularly, relates to a driving circuit to provide driving pulses to the stator without requiring a rotor position detecting element.
2. Description of the Prior Art
In brushless DC motors of the kind normally used in video tape recorders, the stator is typically formed having a three-phase coil with the driving current being sequentially supplied to each coil each time the rotor rotates through an angle of 120.degree.. Generally, a rotor position detector is employed to detect the rotational position of the rotor so that the driving current can be applied to the correct stator coil. When utilizing a brushless DC motor of this kind, timing control is very important to switch properly the flow of current through the three-phase stator coil in phase with the rotor position in order to produce optimum torque. Typically, three individual rotor position detectors are arranged at equal intervals corresponding to the electrical angles of 120.degree.. It is known to utilize magneto-sensitive elements, such as Hall effect devices, for such rotor position detectors.
Use of these individual rotary position detectors is generally troublesome both from the installation and calibration standpoints. Accordingly, it became known to provide rotor detection without the use of physically separate individual rotor detection elements. For example, a brushless DC motor driving circuit that does not employ individual rotor detecting elements is described in U.S. Pat. No. 3,997,823, assigned to the assignee hereof.
While the system described in this patent performs the task of driving the brushless DC motor without the use of separate dedicated rotor position sensors, the circuit used in the system is not one which can be advantageously embodied in an integrated circuit. One reason for this is that the drive circuit described in this patent requires time delay elements that involve the use of capacitors, and capacitors are devices that can not be readily formed in semi-conductor integrated circuits.